A boring tool, or other such underground object, is characterized in a drilling region by six unknown parameters comprising a location in three dimensional space, described using some form of coordinates, and orientation parameters that are generally referred to as pitch, roll and yaw. The former two orientation parameters are rather readily measurable in a direct way, as is typically accomplished in the prior art, by using sensors that are positioned within the boring tool for movement therewith. The yaw orientation parameter, however, is considerably more difficult to determine in the context of prior art techniques, as will be discussed below. Moreover, direct measurement of yaw using, for example, a magnetometer in the boring tool is more problematic than direct measurement of pitch and roll orientation which can be performed with relatively simple mechanical type sensors or based on accelerometers readings. A particular problem resides in such direct yaw measurements being prone to significant levels of measurement error in the presence of magnetic interference.
One class of prior art, as exemplified by U.S. Pat. No. 5,764,062 (hereinafter the '062 patent), entitled TECHNIQUE FOR ESTABLISHING AND RECORDING A BORING TOOL PATH USING A SURVEY REFERENCE LEVEL, simply ignores yaw as an unknown for purposes of position determination. That is, all remaining parameters are integrated to track the underground position of the boring tool.
More recently, another class of system has been developed which accounts for yaw orientation. U.S. Pat. No. 6,035,951 (hereinafter the '951 patent), entitled SYSTEMS, ARRANGEMENTS AND ASSOCIATED METHODS FOR TRACKING AND/OR GUIDING AN UNDERGROUND BORING TOOL, which is commonly assigned with the present application and hereby incorporated by reference, serves as a sophisticated, robust example of such a system. While this system provided remarkable and sweeping advantages over the then-exisiting prior art and continues to be highly effective, one feature is shared with the prior art respecting determination of yaw orientation. Specifically, an electromagnetic locating signal is transmitted from the boring tool which is measured at one or more above ground locations. The measurement of the locating signal then contributes in a direct manner to the determination of yaw orientation. At least in a general sense, the prior art has accepted the precept that measurement of electromagnetic flux is a preferred way to resolve yaw orientation.
The present invention accounts for yaw in all of its various forms and, at the same time, sweeps aside the foregoing precept of the prior art in a highly advantageous manner while providing still further advantages.